1. Field of the Invention
This invention relates to a method for producing a single-crystalline film made of a single crystal of lithium potassium niobate-lithium potassium tantalate solid solution or a single crystal of lithium potassium niobate.
2. Related Art Statement
A device to generate a blue laser is suggested which is made by forming an optical waveguide having a periodically polarization-inversed structure and in which an infrared semiconductor laser is introduced into the optical waveguide (U.S. Pat. No. 4,740,265, JP-A-5-289131, and JP-A-5-173213). For example, JP-A-6-51359 discloses a second harmonic generation (SHG) device in which a polarization-inversed layer, an optical waveguide, a dielectric film, and a reflective grating layer are formed and a thickness of the dielectric film is defined to be a given value.
These techniques require high-precision control of domains in the crystalline film, which makes this techniques difficult to perform. An allowable temperature for the phase-matching must be controlled within a very precise range of xc2x10.5xc2x0 C. Moreover, optical damage of the optical waveguide may be recognized at light energies of 3 mW and higher. Considering these phenomena, it is difficult to practice these techniques to manufacture practical devices.
On the other hand, NGK Insulators, Ltd. suggested in JP-A-8-339002 a SHG device having little optical damage without a quasi-phase-matching or controlling domains to a high-precision. In this literature, a film made of a single crystal of a lithium potassium niobate-lithium potassium tantalate solid solution (often called a xe2x80x9cKLNT single crystalxe2x80x9d) or a single crystal of lithium potassium niobate (often called a xe2x80x9cKLN single crystalxe2x80x9d) was grown by a liquid phase epitaxial growth method.
Furthermore, it is suggested that a single-layered film or a double-layered film made of a single crystal of a lithium potassium niobate-lithium potassium tantalate solid solution be formed on a substrate made of a single crystal of lithium potassium niobate by a metalorganic chemical vapor deposition method (MOCVD method) (JP-A-8-6083). Then, either one of these films is employed as an optical waveguide.
In the case of growing a film of a KLNT single crystal or a KLN single crystal by the above liquid phase epitaxial growth method, the melting point and the Curie temperature of the KLNT single crystal constituting a substrate are about 1000xc2x0 C. and about 500xc2x0 C., respectively. Thus, because growing a single crystal from a melt is required to be within a temperature range of not melting the substrate, the film actually has to be formed in a temperature range of 600xc2x0 C. to 900xc2x0 C. Accordingly, a composition range of the grown film is restricted. Moreover, because the film-forming temperature is normally 600xc2x0 C. to 900xc2x0 C. and higher than the Curie temperature of the substrate, the thus obtained film has a multi-domain structure. Therefore, after growing the single-crystalline film, the film has to be converted to a single domain structure by way of a single-poling step. However this single poling, a crystallinity of the single-crystalline film is to be on one deteriorated and a light propagation loss.
Furthermore, in the MOCVD method, considering decomposition-temperatures of an organic metal oxide of each of K, Li, and Nb, a film has to be formed in a temperature range of 500xc2x0 C. to 800xc2x0 C., so that the film has multi-domain structure. Consequently, the film has to be poled and thus crystallinity of the film is deteriorated.
It is an object of the present invention to obtain a single-crystalline film having small light-propagation loss, in a method for growing a KLN or a KLNT single-crystalline film.
The present inventors conceived that, by a laser ablation method, a laser is irradiated to a target made of a single crystal of lithium potassium niobate-lithium potassium tantalate solid solution, a single crystal of lithium potassium niobate solid solution or a sintered body comprising lithium, potassium, niobium, oxygen etc., and thereby molecules constituting the target were dissociated and evaporated to be gasified, and thereafter the single-crystalline film is epitaxially grown on a substrate made of a single crystal of lithium potassium niobate-lithium potassium tantalate solid solution or a single crystal of lithium potassium niobate solid solution. Consequently, the inventors found that the thus obtained film had excellent characteristics with not much light-propagation loss, and they reached this invention.
Generally, as a vapor phase growing method of a film of a single crystal, a MOCVD method, a MBE method, and a laser ablation method are known. As for the MOCVD method, many studies have been done and various film-formations have been carried out. On the contrary, the laser ablation is applied to a dielectric RAM memory which is produced by film-forming a PZT polycrystal on a silicon-semiconductor, but hardly applied to producing a film of single crystal oxide or producing a film of an optical single crystal.
As an example of applying the laser ablation method to optical single crystals or single crystal oxides, Kawai et al. tried to form a film of lithium niobate (LN) on a lithium tantalate-substrate or a sapphire-substrate (Appl. Phys. Lett. 61(8), 1000(1992), 62, 3046(1993)). However, the laser ablation method does not enable a film of LN having a small light-propagation loss usable for an optical waveguide to be obtained.
For example, a KLNT-single-crystalline film obtained according to the present invention has a higher conversion efficiency of SHG than a KLNT-single-crystalline film obtained according to a conventional MOCVD method does, by 30%, as the light-propagation loss in the single crystal film of the present invention is decreased.
The reason is not clear, but it is presumed that, even though the crystallinity of a substrate is substantially the same, since in the MOCVD method, heating the KLNT-substrate to a relatively high temperature under an oxidizing atmosphere causes lithium atoms and potassium atoms within the KLNT-substrate to be diffused outwardly toward the surface of the substrate and oxidized at this surface, and scattered from the surface, so that the crystallinity in the surface area of the substrate is deteriorated and characteristics of the single-crystalline film are influenced.
Moreover, according to the present invention, the single-crystalline film can be formed under the condition that the substrate is held within a temperature range of 200xc2x0 C. to 600xc2x0 C., particularly preferably 300xc2x0 C. to 500xc2x0 C. during the film formation. Consequently, poling treatment for the single-crystalline film is not required.
Furthermore, in a SHG element, for shortening a phase-matching wavelength of a KLNT-single-crystalline film or a KLN single-crystalline film and increasing its conversion efficiency, a single-crystalline material which is produced by doping rubidium into a KLNT- or a KLN-single crystal is demanded. However, since a rubidium-containing metalorganic compound to be easily handled is not known, a film of a single-crystalline material can not be practically formed by the MOCVD method. According to the present invention, since an oxide powdery mixture, a sintered body, or a single crystal can be freely selected as a target, a KLNT- or a KLN-single-crystalline film containing rubidium can be formed.
The method according to this invention is suitable for manufacturing optical parts, particularly an optical waveguide device. This method is preferred to produce an optical waveguide layer, but also an underclad layer and an overclad layer may be formed by this method.
Following the method of the present invention, the optical waveguide layer can be formed on the underclad layer, and the overclad layer can be formed on the optical waveguide layer. Moreover, following the method of the present invention, the underclad layer, the optical waveguide layer, and the overclad layer can be formed in turn. The term xe2x80x9cunderlayerxe2x80x9d in this specification means an underlayer to form the single-crystalline film directly thereon according to the present invention. For example, the underlayer may be composed of the single-crystalline substrate, the underclad layer, or the optical waveguide layer.